It's a combination of factors, including (1) need for savings and planning for the [non-startup-rich] future, (2) supporting a girlfriend who refuses to live a grad student lifestyle, (3) expensive cost of living in the area I want to live.
I fully realize that most of those sound like excuses more than reasons, but there is definitely a huge chasm between investing money for retirement, buying decent furniture, being able to pay rent and utility bills, etc.; and throwing caution to the wind for a probable failure and extremely strenuous social, economic and physical ordeal.
So I think my answer to why more people don't start startups is rather simple: it's not easy to leap that chasm into poverty, seclusion and exhaustion for a glint of a chance at huge success.
Of course I'll continue hacking on my side projects and hoping my equity in the startup I'm working for affords me the opportunity to take a risk of my own in the near future.